Obsidian hard to get?
With the right world spawn you can have a nether portal within a few minutes.... The only other "tech tree" use is an enchanting table, wich requires 2 diamonds itself aswell as 3 to mine that obsidian. Getting an enchanting table is very doable withing 10 min of world start.
The ignous extruder arguements is pretty meh. Some people think you need diamonds for a nether portal. Others realise all you need is a bucket.
Some people think you need to get sand/tin/copper/iron/redstone and spend time crafting and are then winning the game by moving water and lava around with a bucket. Others simply rush those diamonds for a pick, get that obsidian and get an enchantment table on the side aswell. For early game, an extruder is simply not worth your time.
For endgame. Sure, collecting lava in tanks with a bucket is more convenient then mining it. But compared to just plopping down some water, holding down that mouse button and watching TV. I dunno, I think ill prefer the latter.
Then we, ofcourse, have cross mod interaction. This is what everyone is assuming here. A pump, a chunkloader and voila, infinite obsidian. TE does not provide a pump and does not provide a chunkloader and therefor cannot actually do this. But since we have other mods we might aswell point it the biggest problem here. Its not the extruder, its nether pumping that lava.